Vacuum Cleaner Filter Bag

ABSTRACT

The invention relates to a vacuum cleaner filter bag made of a filter material, comprising at least three layers with at least two layers of a non-woven or non-woven fabric layer, and at least one layer of a fibre web layer of fibres and/or filaments, wherein the at least two non-woven layers and the at least one fibre web layer are connected to each other by a thermal bonding connection.

The present invention relates to a vacuum cleaner filter bag with a filter medium comprising at least three layers, wherein at least two layers consist of a non-woven or non-woven fabric layer, respectively.

In recent years, numerous developments have become known which deal with improving the one- or multi-layered vacuum cleaner filter bags of paper or paper and tissue, respectively, which have been known for a long time in prior art. EP 0 388 479 A1 of Gessner & Co. GmbH describes vacuum cleaner filter bags with an outer filter paper layer and a melt-spun microfibre non-woven (meltblown non-woven).

Multi-layered filter bags of non-wovens (SMS) are disclosed, for example, in U.S. Pat. No. 4,589,894 and in U.S. Pat. No. 5,647,881 of Minnesota Mining and Manufacturing Company (3M). These inventions mainly deal with the improvement of dust recovery.

In EP 0 960 645 A1 and EP 1 258 277 A1 of Airflo Europe N.V., combinations of non-woven layers are described which have a particularly long service life and dust recovery capacity.

EP 1 362 627 A1 of Branofilter GmbH describes filter bags with a multi-layered design where the fibre diameter distributions in the coarse dust filter layer and in the fine dust filter layer are different.

In EP 1 254 693 A2 of Carl Freudenberg K G, a vacuum cleaner bag is described where a preliminary filter layer of a dry-laid, electrostatically charged non-woven is provided in front of a filter layer.

Furthermore, in EP 1 197 252 A1 of 3M Innovative Properties Company, a filter medium of a film fibre non-woven is described which consists of dry-laid, electrostatically charged fibrillated fibres which are connected with each other by ultrasonic bonding to achieve a sufficient strength of the non-woven. Here, it is essential that there are at least two ultrasonic bond points per cm². Thereby, the individual fibres are connected with each other by ultrasonic bonding connections. It is said to be an advantage of such a filter medium that the manufacturing speed is higher compared to that of needling the fibrous web with a scrim and, as in this method the use of a scrim is not necessary, additional air resistance caused by the scrim can be prevented. Moreover, the film fibre non-woven can be connected to further non-woven layers. EP 1 197 252 A1 further discloses to employ this filter medium for air filters. However, the dust storage capacity of this material is not sufficient for the application as filter medium in a vacuum cleaner bag.

Based on the above mentioned prior art, the object of the present invention is to provide a filter bag, the filter material of which has a particularly low bulk density, as compared to those described in prior art, to achieve a superior dust storage capacity. The filter bag is furthermore said to have a design where the structure and the associated advantageous properties of the non-bonded fibre layer are maintained as far as possible.

This object is achieved by a vacuum cleaner filter bag made of a filter material which comprises at least three layers with at least two layers which consist of a non-woven layer, and at least one layer which consists of a non-woven layer of fibres and/or filaments, wherein the at least two non-woven layers and the at least one fibre web layer are connected to each other by a thermal bonding connection.

In this context, the terms fibre web and non-woven in the field of the manufacture of non-wovens are defined and to be understood in the sense of the present invention as follows. For the manufacture of a non-woven, fibres and/or filaments are first laid on a support. Methods of laying are known from prior art. These laid, loose and yet unbound fibres and/or filaments are referred to as fibre web (web). By a so-called non-woven bonding step, finally a non-woven is formed of such a fibre web, the non-woven having sufficient strength to be e.g. wound on reels. This latter non-woven bonding step is thus not performed in the manufacture of the fibre web layer of the invention, instead, the fibre web is bonded to a non-woven layer or between two non-wovens. (Details of the use of the above-mentioned definitions and the above-described methods can be taken from the standard work “Vliesstoffe”, W. Albrecht, H. Fuchs, W. Kittelmann, Wiley-VCH, 2000.)

According to a preferred embodiment of the invention, there are as little thermal bonding connections as possible, relating to the complete area of the filter bag through which a flow can pass. According to the present invention, this is achieved in that, related to the complete area of the filter bag through which a flow can pass, on average maximally 19 thermal bonding connections per 10 cm² are present, preferably maximally 10 thermal bonding connections and particularly preferred maximally 5 thermal bonding connections. The pressing area proportion of the thermal bonding pattern is maximally 5%, preferably maximally 2%, and particularly preferred maximally 1% of the area of the filter bag through which a flow can pass.

In an advantageous embodiment, the filter bag has the additional feature of the average total porosity being at least 65%, preferably at least 80%, particularly preferred at least 95%.

In a further advantageous embodiment, the average median pore diameter is at least 120 μm, more preferred at least 150 μm, even more preferred at least 180 μm, and particularly preferred at least 200 μm. The average median means the arithmetic average value of several measurements of the median of the examined samples.

The measuring method for the determination of the average total porosity and the average median pore diameter according to the present invention is described more in detail with reference to FIGS. 15 to 17.

Due to the fact that a low number of thermal bonding connections is present, the thickness and thus the bulk of the material are clearly increased with the same mass per unit area. Due to the low bulk density of the compound, the material has a high dust storage capacity.

With respect to the geometry, i.e. the distribution of the thermal bonding connections across the area of the filter bag through which a flow can pass, there are no restrictions for the present invention, with the proviso that maximally 19 thermal bonding connections per 10 cm² are present, related to the area of the filter bag through which a flow can pass. The thermal bonding connections can, as a matter of principle, be evenly distributed, i.e. at even distances, across the complete area or else unevenly distributed. The invention thus also includes embodiments where only in predetermined areas there is a higher number of thermal bonding connections and where then larger free areas are formed which are then separated from a next larger free area by an increased number of thermal bonding connections. One essential criterion always is that the given maximal number of thermal bonding connections is not exceeded.

The thermal bonding connections themselves can have different geometries. Thus, punctiform, linear, star-shaped or else bar-shaped thermal bonding connections can be employed. With respect to the precise embodiment of the thermal bonding connections, apart from the number of thermal bonding connections as limiting criterion, only the pressing area proportion of the thermal bonding pattern has to be taken into consideration, which, as already mentioned, is maximally 5%, preferably maximally 2%, and particularly preferred maximally only 1%.

As to the fabric, the fibre web layer of the invention, which is present as a compound with the non-woven layer, comprises all fibres known per se in prior art, in particular staple fibres and/or filaments. Staple fibres in the sense of the invention also include fibrillated film fibres (split fibres) and crimp fibres; here, the staple fibres in the sense of the invention can also be preferably electrostatically charged.

As crimp fibres, in particular those proved to be advantageous which have a spatial structure, such as a zig-zag, wave and/or spiral structure. The advantage of such fibres is that they clearly increase the bulk of the medium.

In this context, the crimp fibre can be a mechanically crimped fibre, an autocrimp fibre and/or a bicomponent crimp fibre. Autocrimp fibres are described e.g. in the EP patent 0 854 943 A1 as well as in PCT/GB00/02998. Bicomponent crimp fibres are available e.g. from the Chisso Corporation in Japan, and crimped spiral-type polyester staple fibres from Gepeco in the USA.

In the invention, staple fibres which are selected from natural fibres and/or synthetic fibres can be employed. Examples of synthetic fibres are in particular polyolefins and polyester. Examples of natural fibres are cellulose, wood fibres, capoc, flax.

With respect to the arrangement of the layers and the number of layers, there are no restrictions for the filter bag in accordance with the invention, with the proviso that at least two layers each consist of a non-woven layer and at least one fibre web layer, wherein these two layers are continuously connected to each other by a thermal bonding, preferably by an ultrasonic bonding connection, as described above.

The non-woven layer of the above described compound is preferably a support or base layer and has a mass per unit area of at least 5 g/m². A scrim is advantageously used for the non-woven layer itself. Here, a scrim means any not airtight material which can serve as base or reinforcing layer. It can be a non-woven, a woven material or a netting. Preferably, it consists of a thermoplastic polymer to facilitate the ability of thermal bonding with the fibre web layer.

Examples of scrims are spunbond fabrics. However, they can also be dry- or wet-laid non-wovens which possess sufficient mechanical stability. The mass per unit area of such a non-woven layer is, according to the present invention, preferably between 10 and 200 g/m², particularly preferred between 20 to 100 g/m². The mass per unit area in g/m² was determined in conformity with DIN EN 29073-1. With respect to the mass per unit area of the fibre web layer, it should be mentioned that the same is to be determined indirectly by means of the compound of non-woven layers and fibre web layer, as the determination of the mass per unit area of the fibre web layer is not possible only on the basis of its loose structure. The determination was therefore effected by a subtraction method, i.e. the mass per unit area of the complete compound, i.e. the compound of non-woven layers and fibre web layer, was determined, and then the mass per unit area of the non-woven layers, which can be determined separately, were subtracted again.

The thickness of the above-described compound of non-woven layer and fibre web layer is between 1 and 7 mm, preferably between 2 and 4 mm. The determination of the thickness was performed according to EDANA 30.5-99, Item 4.2. The apparatus used was a VDM 01 which is available from Karl Schröder K G in Weinheim. As the measurements according to methods 4.1, 4.2 or 4.3 lead to very different results, the measurements of the inventive compounds, i.e. composites, were principally performed according to method 4.2.

The filter bag according to the invention can naturally, as described above, comprise further layers in addition to the compound of the two non-woven layers and the fibre web layer. Preferably, the filter bag according to the invention can furthermore comprise further fine filter layers with different filter properties as required. Here, fine filter spunbond fabric layers are used as fine filter layers. Fine filter spunbond fabric layers in the sense of the invention are corresponding layers which are suited to separate fine particles. Conventional fine fibre spunbond fabric layers are made according to the meltblowing process, the flashspinning process or electrostatic spunbonding. As regards contents, reference is made to the standard work “Vliesstoffe” by W. Albrecht, H. Fuchs, W. Kittelmann, Wiley-VCH 2000, Chapter 4. In the sense of the invention, fine filter layers can also consist of dry-laid non-woven of electrostatically charged fibres.

The filter bag according to the invention is preferably connected by a continuous ultrasonic bonding connection through all layers, i.e. through the non-woven layers and the fibre web layer as well as the further layers. The filter bag according to the invention, however, also comprises embodiments where only thermal bonding connections of the non-woven layers with the fibre web layer are present, and the further layers are connected with the compound of the non-woven layers and the fibre web layer either by gluing or by another connection method.

The invention will be illustrated more in detail below with reference to FIGS. 1 to 14.

FIGS. 1 to 9 schematically show in sections how the filter material of the filter bag according to the invention can be structured.

FIG. 1 shows a two-layered design of a layer 1 in the form of a non-woven layer which is a scrim in FIG. 1. This scrim layer 1 is connected to a fibre web layer 2 by ultrasonic bonding connections. In FIG. 1, the further layer required in accordance with the invention is not depicted.

The structure of the design of the embodiment shown in FIG. 2 essentially corresponds to that of FIG. 1, however with an additional layer of a fine filter medium 3, which in this case represents the third layer. The preferred inflow side is designated with arrows. In this case, the fine filter layer 3 consists e.g. of a meltblown non-woven.

FIG. 3 in turn shows another example, based on FIG. 2, with an additional protective layer 4 which is here arranged at the outflow side. This protective layer 4 can be a scrim, preferably a spunbond fabric.

The embodiment shown in FIG. 4 is of a layer of a non-woven 1 connected to a fibre web layer 2 attached thereto by thermal bonding, as described above, here, however, a layer of a protective non-woven 4 is additionally arranged in front at the inflow side. The non-woven 1 here is in particular a meltblown non-woven.

FIG. 5 differs from FIG. 4 by an additional microfibre non-woven layer 3 arranged at the outflow side.

The example of the structure according to the invention shown in FIG. 6 starts from the design according to FIG. 5, then, however, it has an additional protective layer 4 at the outflow side.

FIG. 7 shows a laminate of two layers of non-woven 1 connected to each other by ultrasonic bonding points between which the fibre web layer 2 is located.

FIG. 8 represents an embodiment of the structure according to the invention based on FIG. 7, here, however, with a layer of a filter medium 3 arranged at the outflow side.

FIG. 9 shows a structure based on FIG. 8 with an additional layer 4 at the outflow side. In FIGS. 1 to 9 described above, the respective structures are only described schematically according to the sequence of layers. The above described structures are then preferably connected by ultrasonic bonding connections.

In Tables 1 to 11 (FIGS. 10 to 12), the measuring results which have been achieved by means of the above described embodiments according to FIGS. 1, 3 and 4 are summarized, in comparison to an embodiment according to EP 1 197 252 A1. In the examples according to FIGS. 1, 3 and 4, a compound which comprises 0.2 thermal bonding points per cm² was employed. In the comparison examples, 2.5 thermal bonding points per cm² were selected. As can be taken from Tables 1 to 11, the materials according to the invention are in particular characterized in that they are thicker than the comparison materials by 15 to 42%. It should be in particular pointed out that this leads to the bulk of the materials according to the invention also being higher by a corresponding amount, namely by 15 to 42%, than in the comparison examples. Now, the superior effect of the materials according to the invention is based on this extremely high bulk, the materials thereby having an above-average dust storage capacity (also see FIG. 14).

FIG. 13 a now shows in the form of a 3D graphic how the low number of thermal bonding points affects the structure of the material. In FIG. 13 a, a material is shown which corresponds to the design according to FIG. 7, i.e. it is a material consisting of a fibre web layer which is connected between two layers of spunbond fabric by ultrasonic bonding connections. In the example according to FIG. 13 a, approx. 0.2 thermal bonding points per cm² were used. FIG. 13 a illustrates the pad-like design leading to the high bulk as described above. In the example according to FIG. 13 a, as fibre web layer 100% of split fibres of polypropylene have been employed. The spunbond fabric also consists of polypropylene. The design of the filter medium represented in FIG. 13 b analogously corresponds to that which has been already described in FIG. 13 a, however with the difference that here 2.5 thermal bonding points per cm² are present. This makes clear that by the design according to the invention in the form of a small number of thermal bonding connections, a clear advantage with respect to the bulk of the material is achieved.

As is now represented in FIG. 14, the embodiment according to the invention leads to a clear increase of the dust storage capacity compared to the filter media as they are described in prior art which comprise 2.5 thermal bonding points per cm². The measuring results represented in FIG. 14 have been performed as follows:

Vacuum cleaner used: Miele Performance 2300

-   -   Typ: HS 05     -   Model: S749     -   No.: 71683038

Performance setting: Maximum

Size of filter bags: 295 mm×270 mm

Dust used for test: DMT Type 8

Test procedure: The dust bag to be tested is incorporated into the apparatus after the apparatus has warmed up for 10 minutes. The flow rate without dust charge is read after a running period of the apparatus of 1 min. Subsequently, the first dust portion of 50 g is sucked in within 30 sec. After 1 min., the present flow rate (in m³/h) is read. This step is correspondingly repeated for the following dust additions, until 400 g of dust have been added.

Filter medium: Spunbond fabric 17 g/m², fibre web 50 g/m²,

-   -   Spunbond fabric 17 g/m²

Thermal bonding

pattern: 1. 2.5 points/cm², evenly distributed

-   -   2. 0.2 points/cm², evenly distributed

The measured values given in the examples were determined by the following determination methods:

Thickness:

30.5-99 Item 4.2 apparatus VDM 01, available from Karl Schröder K G in Weinheim.

As the measurements according to methods 4.1, 4.2 or 4.3 lead to very different results, the measurements of the laminates according to the invention were in principle performed according to method 4.2 (for bulky nonwovens with a maximum thickness of 20 mm).

Mass per unit area [g/cm²]: DIN EN 29073-1

Bulk [cm³/g]:

Thickness (EDANA 30.5-99 Item 4.2)/mass per unit area (DIN EN 29073-1)

Bulk density [g/cm³]:

Mass per unit area (DIN EN 29073-1)

Thickness (EDANA 30.5-99 Item 4.2)

In FIG. 15, the principle of measurement for the determination of the average total porosity and the average median pore diameter are schematically illustrated.

FIG. 16 shows a device which is employed for the determination of the average total porosity and the average median pore diameter.

Table 9 (FIG. 17) represents the measured values with respect to the average total porosity and the average median pore diameter.

The measured values were determined according to the method given below.

To determine the average total porosity and the average median pore diameter, the methodology of the extrusion of a wetting liquid was used. The measurements were performed by means of a PMI liquid extrusion porosimeter. Below, reference will be made to FIGS. 15 and 16.

1. Principle of Measurement

As the differential free surface energy of the system wetting liquid 20/sample 12 is lower than the differential free surface energy of the system air/sample 12, the pores of a sample spontaneously fill with wetting liquid 20. The wetting liquid 20 can be removed from the pores by increasing the differential pressure 22 of an inert gas 18 on the sample 12. It was shown that the differential pressure 22 required to displace the wetting liquid 20 from a pore is determined by the size of the pore (Akshaya Jena, Krishna Gupta, “Characterization of Pore Structure of Filtration Media”, Fluid Particle Separation Journal, 2002, 4 (3), p. 227-241). The correlation between the differential pressure 22 of the inert gas 18 and the pore size is represented by equation 1.

p=4γ cos θ/D  (1)

wherein p is the differential pressure 22 of an inert gas on the sample, γ is the surface tension of the wetting liquid 20, θ is the contact angle of the wetting liquid 20 on the pore surface, and D is the pore diameter the definition of which is quoted for an irregular cross-section by the following equation (2).

D=4(cross-sectional area)/(cross-sectional circumference)  (2)

If the sample 12 is applied to a membrane 25 and the pores of the sample 12 and the membrane 25 are filled with a wetting liquid 20, the application of a pressure 23 on the sample 12 leads to a displacement 23 of the liquid from the pores of the sample 12 and to a flowing out 24 of the liquid 20 through the membrane 25. If the largest pore of the membrane 25 is smaller than the smallest pore of interest of the sample 12, the liquid 20 is displaced from the pores of interest of the sample 12 and will flow out of the membrane 25, however, the pressure 22 will not be sufficient to completely remove the liquid 20 from the pores of the membrane 25, the gas will not be able to flow through the pores of the membrane 25 filled with liquid and out of the same. Thus, by means of the differential pressure 22 and the flown-out volume of the liquid 20, the diameter or the volume of the pores, respectively, can be determined (A. Jena und K. Gupta, “A Novel Technique for Pore Structure Characterization without the use of Any Toxic Material”, Nondestructive Characterization of Materials XI, Ed.: Robert E. Green, Jr., B. Boro Djordjevic, Manfred P. Hentschel, Springer-Verlag, 2002, p. 813-821).

2. Experimental Setup

Basis of the PMI liquid extrusion porosimeter 5 (FIG. 16) is the methodology of liquid extrusion. The sample chamber 6 of the porosimeter 5 consists of a cylindric PVC container the diameter of which is 45 mm and the depth of which is 45 mm. A relatively wide meshed open netting 7 made of rust-resistant steel wire rests on a strip at the bottom of the sample chamber 6. Underneath the netting 7, the sample chamber 6 is connected to the bottom side of a cylindric acrylic vessel of which the diameter is 40 mm and the depth is 40 mm, by means of a flexible tube 8 having a diameter of only a few mm. The vessel 9 as well as its cover 10 are placed on a pair of scales 11 (manufacturer: Mettler, weight resolution 0.0001 g). A cylindric inset 13 (40 mm diameter, 40 mm height) is placed on the sample 12 within the sample chamber 6. The upper side of the inset 13 comprises a notch for an O-ring 14. A pneumatically operated device 15 which comprises a piston 16 guided in a cylinder is mounted on the sample chamber 6. The piston 16 is hollow to ensure a flow of the test gas 18 into the sample chamber 6. A flat disk 17 of rust-resisting steel which is thermally bonded to the bottom side of the piston 16 presses the inset 13 against the O-ring 14 on the upper side of the inset 13 and thus prevents the test gas 18 from escaping. The piston 16 is controlled pneumatically. The test gas 18 and the gas 19 for operating the piston 16 are supplied separately.

3. Wetting Liquid

In all tests, Galwick, which is a perfluorinated polymer (oxidized and polymerized 1,1,2,3,3,3-hexafluoropropene), was used as wetting liquid. The liquid is inert, the surface tension is 16 Dynes/cm. Due to the very low surface tension of the test liquid, the contact angle is near 0° (Vibhor Gupta and A. K. Jena, “Substitution of Alcohol in Porometers for Bubble Point Determination”, Advances in Filtration and Separation Technology, American Filtration and Separation Society, 1999, 13b, p. 833-844).

4. Test Gas

In all tests, dry and purified compressed air was used. To remove solid particles, the air was filtered, the moisture was removed by the standard drying methods known to the expert from prior art.

5. Automated Test Performance, Data Acquisition and Management

The test performance, data acquisition as well as the data reduction were performed in a completely automated manner by using a computer and appropriate software. The test procedure was performed automatically after the sample chamber 6 had been charged with a sample 12, so that accurate and reproducible results could be obtained.

6. Test Procedure

a) Preparation of the Measuring Instrument

The sample chamber 6, the vessel 9 on the pair of scales 11, the netting 7 at the bottom of the sample chamber 6 and the inset 13 were cleaned with alcohol to remove impurities. The O-rings 14 were also cleaned and greased. A Millipore membrane 25 with a maximum pore diameter of 0.45 μm was placed on the netting 7. It has to be taken care that the membrane 25 is undamaged, i.e. that it does not comprise any defects, cracks or other damages, as this could otherwise lead to falsified measuring results. Now, wetting liquid 20 which flows into the sample chamber 6 via the tube 8 was put into the vessel 9. In the process, enough wetting liquid 20 was added to reach a liquid level in the sample chamber 6, such that the liquid 20 just covers the netting 7. This ensures a complete wetting of the membrane. After a certain period, a constancy of the indication of the scales 11 was reached, indicating that a stationary state was reached.

b) Preparation of the Samples

For the measurement, filter bags were used which were made of a filter bag material consisting of a compound of a fibre web layer enclosed between two non-woven layers. The non-woven layers (spunbond layers) are formed of polypropylene fibres. The fibre web layer consists of polypropylene staple fibres (split fibres having a length of 60 mm). The filter material is connected by point-like thermal bonding connections which are introduced by means of ultrasonic bonding. Three samples were examined which had different numbers of thermal bonding points, namely 16, 70 and 95, in each case related to 100 cm², which are evenly distributed across the surface. Then, circular samples 12 having a diameter of 45 mm were punched out of the filter bags. The samples 12 were weighed and the thickness was determined according to EDANA 30.5-99 Item 4.2 (cf. p. 8, 1. 3-13). It was difficult to make any statements about the thickness which is due to the soft nature and the uneven surface of the sample 12. The bulk density ρ_(b) was calculated. This bulk density corresponds to that of the dry sample. The upper layer of the sample 12 was scratched with a knife (Stanley knife). Each cut had a length of 10 mm and a width of 1 mm. To find out an adequate number of cuts, samples 12 having different numbers of cuts were examined. Based on the results obtained with these samples 12, it was found out that five cuts per sample 12 are adequate; thus, all examinations were performed with five cuts per sample 12. The five cuts were arranged analogously to the arrangement of the points of a five on a dice.

c) Wetting and Charging of the Sample

The sample was placed into a vessel containing wetting liquid 20. In the process, the sample 12 absorbed the wetting liquid 20 and showed a tendency to swell. It was taken care that the sample 12 was not completely immersed in the liquid 20 to avoid inclusions of air in the sample 12. The wetted sample 12 was subsequently placed on the membrane 25 within the sample chamber 6. The O-ring 14 was placed on the sample 12 and the inset 13 on the O-ring 14.

d) Performance of the Test

All information concerning the sample 12 including the identification number were stored in a computer. The units as well as the various functions to be measured were also entered. Subsequently, the test was carried out.

The piston 16 was lowered by computer control to press the inset 13 onto the O-ring 14. To avoid leakages, a predetermined pressure was applied on the O-ring 14. The scales 11 were tared. Subsequently, the test gas 18 was slowly introduced through the piston 16 to the surface of the sample 12. The gas pressure 22 was computer controlled, increased in small increments, thus an adjustment of a balance of the system was achieved before the data were recorded. The computer stored the data of the pressure and the change of weight of the liquid by means of the scales 11. The results were also graphically represented to follow the progress of the test. To obtain the results at the end of the test, the data were printed in various ways.

7. Results

The measuring device 5 recorded the weight increase of the wetting liquid 20 which was displaced from the sample 12 by means of the scales 11 and converted the weight of the liquid 20 by means of the density to the corresponding volume. This result represents the cumulative pore volume. Equally, the pore diameter was calculated from the gas pressure of the test gas 18 determined by the measuring device 5, which test gas was used to displace the wetting liquid 20 from the pores of the sample 12. Thus, the cumulative pore volume could be recorded as function of the pore diameter. The porosity P (in %) was calculated from the bulk density Pb and the total pore volume V according to equation (3).

P=(Vρ _(b))×100  (3)

By means of the measuring device 5, the median pore diameter could also be calculated. The median pore diameter is defined such that 50% of the complete pore volume come from pores which are larger than the mean pore, and 50% of the complete pore volume come from pores which are smaller than the mean pore. The arithmetic average of several measurements of the used samples is given in Table 9 (FIG. 17) as average median pore diameter. As can be taken from Table 9, the filter material of the bag according to the invention has an extremely high average total porosity of up to 96.8%. With an increasing number of thermal bonding connections, the total porosity then falls to a value of 67.4%. Correspondingly, the average median pore diameter falls from 201.8 μm to 129.1 μm. As the results indicate, the filter bags according to the invention have an extremely high porosity, which finally leads to an above-average dust storage capacity.

8. Discussion of the Measurement Method

In the measurement methodology used, the pore diameter and the pore volume of a sample are calculated from the measured gas pressure required to displace the wetting liquid from the pores, as well as from the measured volume of the liquid displaced from the pores. The pores in the non-woven layers (spunbond layers) of the sample attached at the top and at the bottom are much smaller than the pores of the fibre web layer in the central layer. From equation 1 one can see that the gas pressure required to displace a liquid from the layers placed at the top and at the bottom has to be much higher than that which is required for the fibre web layer. In the examination of the filter bags, a displacement of the liquid 20 from the pores of the central fibre web layer will not occur before the liquid has been displaced from the pores of the spunbond non-woven layer attached at the top. The high pressure required to displace the liquid from the small pores of the spunbond non-woven layer attached at the top will also displace liquid from the larger pores of the central fibre web layer. Thus, the diameter of the small pores of the spunbond non-woven layer attached at the top is measured as the diameter of the pores in the fibre web layer as central layer. The determined pore volume will be close to the pore volume of the central layer as the volume of the small pores in the very thin layers attached at the top and at the bottom is negligible, compared to the large volume of the large pores in the thick central layer.

The test procedure used in this examination also includes the introduction of several cuts on the upper layer. By the cuts, large openings were included in the upper layer such that the test gas could pass the small pores of the upper layer. In the process, the diameter and the volume of the small pores in the upper layer were not measured. Thus, the liquid was displaced from the central layer with small pressures which correlate with the large pores in the fibre web layer. The spunbond non-woven layer attached as lower layer did not have any influence on the test as the liquid which was displaced from the pores of the fibre web layer by means of gas pressure simply flowed through the lower spunbond non-woven layer, and the gas pressure was thus not suited to displace liquid from the lower layer. Thus, the diameter and the volume of the pores in the fibre web layer were determined with this test. 

1. Vacuum cleaner filter bag of a filter material comprising at least three layers with at least two layers of a non-woven or non-woven fabric layer, and at least one layer of a fibre web layer of fibres or filaments, wherein the at least two non-woven layers and the at least one fibre web layer are connected to each other by a thermal bonding connection.
 2. Filter bag according to claim 1, wherein a pressing area proportion of a thermal bonding pattern is maximally 5% of a surface of an area of the filter bag through which a flow can pass, and on average maximally 19 thermal bonding connections per 10 cm² are present.
 3. Filter bag according to claim 1, wherein an average total porosity is at least 65%.
 4. Filter bag according to claim 1, wherein an average median pore diameter is at least 120 μm.
 5. Filter bag according to claim 1, wherein on average maximally 10 thermal bonding connections per 10 cm² are present.
 6. Filter bag according to claim 1, wherein the thermal bonding connection has a star-shaped, punctiform, bar-shaped or linear design.
 7. Filter bag according to claim 1, wherein a pressing area proportion of the thermal bonding pattern is maximally 2%.
 8. Filter bag according to claim 1, wherein the fibres comprise a form of staple fibres, and wherein the staple fibres comprise split fibres or crimp fibres.
 9. Filter bag according to claim 8, wherein the fibres have a length of between 1 and 100 mm.
 10. Filter bag according to claim 8, wherein the crimp fibres have different spatial structures, comprising a zig-zag, wave or spiral type.
 11. Filter bag according to claim 8, wherein the crimp fibres comprise a form of mechanically crimped fibres or autocrimp fibres or bicomponent fibres.
 12. Filter bag according to claim 8, wherein the fibres are electrostatically charged.
 13. Filter bag according to claim 8, wherein the staple fibres comprise a form of natural or synthetic fibres.
 14. Filter bag according to claim 1, wherein a mass per unit area of the fibre web layer is between 10 and 200 g/m².
 15. Filter bag according to claim 1, wherein the fibre web layer has a mass per unit area of at least 5 g/m².
 16. Filter bag according to claim 15, wherein the non-woven layer is a scrim.
 17. Filter bag according to claim 1, comprising two non-woven layers between which the fibre web layer is arranged.
 18. Filter bag according to claim 17, wherein at least one non-woven layer comprises a fine filter spunbond fabric layer.
 19. Filter bag according to claim 1, further comprising a fine filter spunbond fabric layer.
 20. Filter bag according to claim 19, wherein at least one non-woven layer comprises a second fine filter spunbond fabric layer and wherein the fine filter spunbond fabric layers comprise different filter properties.
 21. Filter bag according to claim 1, comprising a further layer comprising paper, non-woven material or nanofibres. 